The reforming of hydrocarbons to produce a mixture of hydrogen, carbon monoxide, carbon dioxide, hydrocarbons, and water, which is called synthesis gas (syngas), has been practiced by various industries for a long time. Reforming can be done with steam (known as steam reforming, endothermic) to produce syngas with high H2 to CO ratios, with oxygen for partial oxidation of methane (called partial oxidation, exothermic), and with carbon dioxide (known as carbon dioxide reforming, endothermic) for production of syngas with low H2 to CO ratios. Syngas produced by the reforming of hydrocarbons in these manners has been widely utilized in the production of ammonia, methanol, hydrogen, liquid fuels, oxygenated compounds, etc.
Hydrogen generated from the reforming of hydrocarbons has also been used as fuel in fuel cells where hydrogen and oxygen react to form water. In this capacity, they generate electricity with a much higher efficiency than when compared to their conventional usage as fuels for energy purposes. In certain cases, such as proton-exchanged membrane fuel cells, hydrogen must be extremely pure in order to be utilized as fuel. However, for Molten Carbonate Fuel Cells, hydrogen in the mixture of syngas can be directly utilized as fuel to generate electricity; carbon dioxide and water molecules do not need to be removed from the gas stream. Therefore, syngas containing hydrogen for molten carbonate fuel cells is usually produced in-situ either by external reforming or internal reforming. Internal reforming for molten carbonate fuel cells can be carried out in two different methods: direct internal reforming and indirect internal reforming.
In molten carbonate fuel cells, LiAlO2 carrying electrolytes (mixture of Li2CO3, Na2CO3, and K2CO3) often cause electrical resistance in the electrolyte matrix. During power generation, internal electrical resistance in molten carbonate fuel cells generates undesirable heat. This undesirable heat must be removed in order for the fuel cells to remain at an operational temperature. In addition, the reforming of hydrocarbons to produce syngas is an endothermic reaction system that requires external heat to sustain the catalytic reactions. Therefore, it is highly advantageous and efficient to adapt internal reforming in order to use the undesirable heat generated from fuel cells to heat the reactor of hydrocarbon reforming for the production of hydrogen as fuel for fuel cells.
There are difficulties in the direct internal reforming of hydrocarbons for molten carbonate fuel cells that arise from the contamination of reforming catalysts via constant diffusion and deposition of the electrolyte vapors of Li—Na—K hydroxide(s) and/or carbonate(s). This electrolyte deposition continuously deactivates the reforming catalysts throughout the life of catalyst usage, which results in a shortened catalyst life.
At the same time, the catalytic reforming of hydrocarbons is usually carried out at temperatures ranging from 300° C. to 900° C., even up to 1000° C. The presence of both heat and steam leads to aging of the reforming catalysts, a loss of surface area for active components and/or support materials, and is sometimes accompanied by phase transformation. Losing the surface area of the active components leads to the loss of the catalytic activity of hydrocarbon reforming, which also results in a shortened catalyst life.
The presence of sulfur-containing molecules in the hydrocarbon stream leads to the deposition of sulfur-related chemicals. Reforming catalysts are usually sensitive to the deposition of sulfur-related chemicals, which leads to the deactivation of the reforming catalysts. Therefore, the sulfur content in the feedstock of hydrocarbons is usually removed to a level of less than 100 ppb; most often, the sulfur content must be reduced to only a few ppb for viable usage in fuel cells. Sulfur poisoning from the hydrocarbon feeds can also result in a shortened catalyst life.
Another factor that causes deactivation of the reforming catalyst is coke formation (carbon deposit) on the reforming catalysts. The presence of steam at a relatively high temperature usually eliminates or minimizes the issue of coking.
A combination of deactivations caused by the presence of heat, the presence of steam, and the deposition of electrolytes causes reforming catalysts to lose their capacity for activity until eventually, they are no longer efficient enough to allow the fuel cells to function normally. Extending the life of reforming catalysts is a key challenge in the development of molten carbonate fuel cells with prolonged operational lifespan. A longer lifespan for molten carbonate fuel cells allows for a more efficient, economical, and environmentally sound method of energy production.
The current invention concerns new catalysts that are able to sustain prolonged catalyst life as reforming catalysts as a result of preferred compositions and pore structures.